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Goal

Determine appropriate amplifier for a photomultiplier tube to be used in a detector, so as to prevent the background noise from drowning out the signal from the photomultiplier tube.

Challenges

Slew Rate

The slew rate is defined as the measurement of how fast an amplifier can respond to an electrical signal. Thus, the slew rate determines the range of frequencies that can be amplified according to the formula S = 2πfV where f is frequency and V is peak voltage. In order for our amplifier to work with the photomultiplier tube, the slew rate must be high enough to accurately amplify the signal. The slew rate is expressed in volts per second, but more commonly volts per microsecond.

Cost

While not necessarily a major concern, it would be in the best interest of the program to find an optimal solution that gives good results with as little cost as possible.

Amplifiers

There are two models of amplifiers we will test for usage in the detector: the LM 741C operational amplifier and the LMH 730154 high speed differential amplifier.

741C Operational Amplifier

Pros: Cheap ($0.75), very durable

Cons: Low slew rate, amplification bandwidth only extends to about 1 MHz

LMH 730154 Differential Amplifier

Benefits

The slew rate for the LMH 730154 Differential Amplifier is very high and has an amplification bandwidth of approximately 2 GHz. This means that the amplifier can properly amplify signals of up to 2 GHz without any major signal distortions.

Photomultiplier Tubes (PMT)

Basic Operation Principle

A photomultiplier tube (PMT) is a device that has the capability of detecting single photons and small pulses of light. The underlying principle governing the workings of the device is the photoelectric effect. The photoelectric effect is simply the action of a photon (with sufficient energy) knocking an electron out of orbit. This electron is then multiplied by a series of plates called dynodes. The dynodes are at a positive voltage so that the electron is accelerated towards the plate. When the electron collides with the dynode a shower of electrons are emitted. These electrons then are accelerated towards the other dynodes and it also emits a shower of electrons. This avalanche effect results in thousands and sometimes millions of electrons being liberated. These are the electrons that are detected as an electrical pulse.

PMT Gain

The gain of a PMT is directly proportional to the voltage applied to the dynodes and the number of dynodes. The gain of a PMT usually ranges from one million to one hundred million. This large gain is sometimes large enough to be detected directly, however tests will have to be run in order to test if this claim is true. If the gain is sufficiently large, then no amplifier is required. If it is not, an amplifier will be required.

Procedure

We will begin by testing the PMT without an amplifier to see if we can get a discernible signal consistently, which could potentially allow us to skip using an amplifier altogether. If this course of action does not pan out, we will move on to testing the PMT with both amplifiers and comparing the data. This should help us see which amplifier is the right fit for this usage.